19.6. Extreme and Irreversible Effects

19.6.1. The Irregular Face of Climate Change

Natura non facit saltusnature does not take jumps. Modern science has
thoroughly shattered this tenet of the Aristotelian school of thought. Long-term
observations and experimental insights have demonstrated convincingly that smooth,
or regular, behavior is an exception rather than a rule. Available records of
climate variability, for example, reveal sudden fluctuations of key variables
at all time scales. Large, abrupt climate changes evident in Greenland ice-core
records (known as Dansgaard-Oeschger oscillationsDansgaard et al., 1993)
and episodic, massive discharges of icebergs into the North Atlantic (known
as Heinrich eventsBond et al., 1992) are obvious examples of irregular
behavior as a result of weak external forcing. Ecosystems also display discontinuous
responses to changing ambient conditions, such as changes in disturbance regimes
(Holling, 1992a; Peterson et al., 1998) and species extinctions (Pounds et al.,
1999). Irreversible changes in ecosystems are triggered by disturbances (e.g.,
Gill, 1998), pests (e.g., Holling, 1992b), and shifts in species distributions
(Huntley et al., 1997). Irregular behavior is accepted as a major aspect of
the dynamics of complex systems (Berry, 1978; Schuster, 1988; Wiggins, 1996;
Badii and Politi, 1997).

A quantitative entity behaves "irregularly" when its dynamics are
discontinuous, nondifferentiable, unbounded, wildly varying, or otherwise ill-defined.
Such behavior often is termed singular, particularly in catastrophe theory (Saunders,
1982), and illustrates how smooth variations of driving forces can cause abrupt
and drastic system responses. The occurrence, magnitude, and timing of singularities
are relatively difficult to predict, which is why they often are called "surprises"
in the literature.

It is important to emphasize that singular behavior is not restricted to natural
systems. There has been speculation, for example, about possible destabilization
of food markets, public health systems, and multilateral political agreements
on resource use, but solid evidence rarely has been provided (e.g., Döös,
1994; Hsu, 1998). Rigorous scientific analysis of certain classes of singular
socioeconomic phenomena is emerging (Bunde and Schellnhuber, 2000), but huge
cognitive gaps remain in this field.

Singularities have large consequences for climate change vulnerability assessments.
Unfortunately, most of the vulnerability assessment literature still is focusing
on a smooth transition from what is assumed to be an equilibrium climate toward
another equilibrium climate (often 1xCO2 to 2xCO2). This
means that most impact assessments still implicitly assume that climate change
basically is a "well-behaved" process. Until recently, only a few
authors have emphasized the importance of discontinuous, irreversible, and extreme
events to the climate problem (e.g., Lempert et al., 1994; Nordhaus, 1994a;
Schellnhuber, 1997); concerns about the impacts of these events and their consequences
for society now are becoming much more common. Singularities could lead to rapid,
large, and unexpected impacts on local, regional, and global scales. Anticipating
and adapting to such events and their impacts would be much more difficult than
responding to smooth change, even if these responses must be made in the face
of uncertainty. Furthermore, singularities considerably complicate the search
for optimal emissions reduction strategies that are based on, for example, benefit-cost
analysis or tolerable emissions strategies that are based on, as another example,
the precautionary principle.

This section reviews and synthesizes relevant available information on the
impacts of singular behavior of (components of) the climate system or singular
impacts of climate change and draws conclusions about the consequences for vulnerability
assessments. Because no generally accepted framework to assess singularities
of climate change exists, an illustrative typology of singularities is discussed
first. The different characteristics of each class in this typology justify
why insights from this section contribute to two separate reasons for concern:
extreme weather events and large-scale singularities.

19.6.2. Characteristics of Singularities

The causes of singularities are diverse, but most can be grouped in the categories
of nonlinearity, complexity, and stochasticity. Choices about how to assess
singular climate impacts depend strongly on the factors generating such behavior.
The first two categories arise in a largely deterministic context, so their
incidence can be assessed with proper models. The latter is probabilistic, however,
rendering its incidence basically unpredictable. Only statistical properties
can be analyzed. Predictability (and consequently adaptability) is directly
related to the stochastic nature of the underlying dynamics.

The first, and most obvious, class of singularities is caused by strongly nonlinear
or discontinuous functional relationships. A conspicuous case is the critical
threshold, where responses to a continuous change in a driving variable bring
about sudden and severe impacts, such as extinction events. Changes in mean
climate can increase the likelihood of crossing these thresholds. Even one of
the simplest physical thresholds in the climate systemthe melting point
of icecould induce singular impacts. For example, thawing of permafrost
regions would be induced by only a few degrees of warming (Pavlov, 1997) and
would severely affect soil and slope stability, with disastrous effects on Arctic
infrastructure such as oil pipelines (see Section 16.2.5
and SAR WGII Section 11.5.3). Section 19.3 extensively
illustrates the occurrence of critical thresholds that are relevant for bleaching
of coral reefs (a temperature threshold) and coastal mangroves (a sea-level
rise threshold).

Complexity itself is a second potential cause for singular behavior in many
systems. Complex systems, of course, are composed of many elements that interact
in many different ways. Anomalies in driving forces of these systems generally
distort interactions between constituents of the system. Positive feedback loops
then can easily push the systems into a singular response. (Note that complexity
is by no means synonymous with nonlinearity!)

Complex interactions and feedbacks gradually have become a focal point of global
and climate change investigations: Several illustrative studies, for example,
deal with the interplay between atmosphere, oceans, cryosphere, and vegetation
cover that brought about the rapid transition in the mid-Holocene from a "green"
Sahara to a desert (Brovkin et al., 1998; Ganopolski et al., 1998; Claussen
et al., 1999), with the mutual amplification of regional climate modification
and unsustainable use of tropical forests as mediated by fire (Cochrane, 1999;
Goldammer, 1999; Nepstad et al., 1999) and with the dramatic disruptions possibly
inflicted on Southern Ocean food webs and ecological services by krill depletion
resulting from dwindling sea-ice cover (see Brierly and Reid, 1999; see also
Section 16.2.3).

The third category, stochasticity, captures a class of singularities that are
triggered by exceptional events. In the climate context, these are, by definition,
extreme weather events such as cyclones and heavy rains (see Table
3-10). Their occurrence is governed by a generally well-behaved statistical
distribution. The irregular character of extreme events stems mainly from the
fact that, although they reside in the far tails of this distribution, they
nonetheless occur from time to time. Therefore, they could affect downstream
systems by surprise and trigger effects that are vastly disproportional to their
strength. Climate change also could lead, however, to changes in probability
distributions for extreme events. Such changes actually could cause serious
problems because the risk and consequences of these transitions are difficult
to quantify and identify in advance. The impacts caused by these events have
not yet been explored, although they should constitute an essential aspect of
any impact and adaptation assessment.

The impacts of extreme event consequences of stochastic climate variability,
however, have begun to attract researchers' attention in a related context.
As noted in Chapter 18, changes in mean climate can increase
the likelihood that distributed weather will cross thresholds where the consequences
and impacts are severe and extreme. This variant of stochastic singularity therefore
can change in frequency even if the probability of extreme weather events, measured
against the mean, is unaffected by long-term trends.

There also is a fourth type that generally arises from a combination of all
other singularity categories. This typesometimes referred to as "imaginable
surprises" (Schneider et al., 1998; see also Chapter
1)represents conceivable global or regional disruptions of the operational
mode of the Earth system. Such macro-discontinuities may cause damages to natural
and human systems that exceed the negative impacts of "ordinary" disasters
by several orders of magnitudes.

Responses to climate change can alter their character from singular to regularand
vice versaas they cascade down the causal chain: geophysical perturbations,
environmental impacts, sectoral and socioeconomic impacts, and societal responses.
Only the last three are climate change effects in the proper sense, but the
first is important because it translates highly averaged indicators of climate
change into the actual trigger acting at the relevant scale. Most singular geophysical
perturbations create singular impactswhich may, in turn, activate singular
responses. One therefore might assume that singularities tend to be preserved
down such a cascade. Singular events also can arise further down the causal
chain. Purely regular geophysical forcing, for example, can cause singular impacts,
and singular socioeconomic responses may result from regular impacts.

Harmful impacts of climate change generally can be alleviated by adaptation
or exacerbated by mismanagement (see, e.g., West and Dowlatabadi, 1999; Schneider
et al., 2000a; see also Chapter 18). Climate-triggered
singular phenomena can generate substantial impacts because their predictability
and manageability are low. Such impacts would be considerably reduced if they
could be "regularized" by appropriate measures. For example, an ingenious array
of seawalls and dikes could transform an extreme storm surge into a mundane
inundation that could be controlled by routine contingency procedures. So too
could a long-term policy of retreat from the sea. However, inappropriate flood
control structures could wreak havoc, particularly because they foster a false
sense of security and actually inspire further coastal development.

In summary, singularities tend to produce singularities, as a rule; regularities
may turn into singularities under specific conditions, and singularities can
be regularized by autonomous ecological processes or judicious societal measures.
Defining the propagation of singular events in the causal cascade or opportunities
to convert them into regular events remains a major research challenge.